The present invention relates to methods for removing water and impurity metals from gases, in particular acid gases, using a zeolite purifier with a high silica-to-alumina ratio, and with low impurity metal levels, especially low titanium levels, which has been heated to at least about 350xc2x0 C. but preferably to at least about 650xc2x0 C. The invention also relates to methods for preparing zeolites with low levels of impurity metals.
A wide range of hydridic, halide and bulk gases are used in processes for manufacture of semiconductor devices and materials. As semiconductor geometries have become smaller and devices more sophisticated, the purity of these gases has become more crucial to the viability and success of semiconductor manufacture.
Water contamination in acid gases used in the production of semiconductors is particularly disadvantageous for a number of reasons. Even trace amounts of water in acid gases such as hydrogen chloride (HCl) and hydrogen bromide (HBr) cause corrosion of the piping, valves and flowmeters used to handle the gases in semiconductor manufacture. The presence of water in these gases can also cause the walls of the cylinders used to store the gases to corrode. Such corrosion leads to the generation of metal particulate contaminants which can become incorporated into the semiconductor device during manufacture. In addition, certain processes used in semiconductor manufacture result in the decomposition of water present in the process gas into H2 and O2. The presence of these gases can result in formation of additional contaminants, particularly oxides, which can also become incorporated into the semiconductor device. Contamination of semiconductor devices with metal particulate and oxide impurities is severely detrimental to the performance of the devices, and often renders the devices deficient or even useless for their intended purpose. Moreover, the corrosion caused by the presence of water in these gases necessitates frequent replacement of expensive piping, manifolds, valves and other gas handling equipment.
Metallic impurities in acid gases can also have a detrimental effect on semiconductor manufacturing processes. Metallic impurities, such as titanium, iron, magnesium, zinc, calcium or aluminum, can become incorporated into the semiconductor devices during manufacture. Contamination of semiconductor devices with such metallic impurities can cause xe2x80x9cshortsxe2x80x9d or xe2x80x9copensxe2x80x9d in the microelectronic circuit, rendering the semiconductor device inoperative. In addition, metallic contamination in acid gases can cause xe2x80x9chazexe2x80x9d or xe2x80x9cspikesxe2x80x9d on wafers after polishing.
A number of materials have been developed for the removal of moisture from acid gases. One such material is a chlorosilylated alumina which is effective for removal of trace moisture from hydrogen chloride, hydrogen bromide, chlorosilanes and chlorine. This material comprises an octahedral alumina substrate with Alxe2x80x94Oxe2x80x94Al linkages, which is functionalized with chlorosilyl groups. The material removes water from the gas by an irreversible chemical reaction of the surface chlorosilyl groups with water, and is capable of removing moisture to levels below 0.1 ppm.
There are a number of disadvantages associated with the use of chlorosilylated alumina for removal of trace moisture from acid gases. The preparation of this material is complex and expensive, involving treatment with silicon tetrachloride (SiCl4), which is a corrosive material. Moreover, chlorosilylated alumina is only suitable for applications using low pressure HCl, i.e., about 50 psig or less. At high pressure, the HCl reacts with the alumina, producing aluminum trichloride (AlCl3 or the dimer, Al2Cl6) which contaminates the purified gas stream. In the case of HBr, contamination with the aluminum halide occurs even at low pressure since HBr is more reactive than HCl and AlBr3 (Al2Br6) is more volatile than AlCl3 by about an order of magnitude. The leaching of aluminum from chlorosilylated alumina purifiers in this manner causes the structure of the chlorosilylated alumina to degrade, resulting in particulate contamination of the gas, and necessitating frequent replacement of this solid purifier. Moreover, the material requires a preconditioning step with the halide acid gas during which water is initially generated, with a concomitant temperature increase to 120-150xc2x0 C. This preconditioning step is time consuming and requires the use of a significant quantity of costly halide acid gas. Furthermore, in many applications, the preconditioning must be conducted off-line, so that critical downstream components are not damaged by the initial surge of moisture from the purifier.
Alumino-silicate zeolites, in particular, molecular sieves of the Zeolite A family such as the 3A, 4A and 5A zeolites, are well known moisture adsorbents. However, the Zeolite-A molecular sieves have proved to be unsuitable for drying acid gases such as HCl and HBr. See, e.g., Barrer, R. M. and Kanellopoulos, A. G., 1970, xe2x80x9cThe Sorption of Ammonium Chloride Vapor in Zeolites. Part I. Hydrogen Chloride and Ammonia,xe2x80x9d J. OF THE CHEM. SOC. (A):765 (decomposition of 4A molecular sieves was observed upon exposure to hydrogen chloride at a pressure of 228 mm Hg for 18 hours at 50xc2x0 C.). The stability of the alumino-silicate zeolites to hydrogen chloride has been found to relate to the silica-to-alumina ratio. The higher the silica-to-alumina ratio, the more stable the zeolite is to hydrogen chloride, with zeolites having silica-to-alumina ratios of 10 and above being considered sufficiently stable to HCl. In contrast, the Type A and Type X (synthetic faujasite) zeolites have silica-to-alumina ratios of 2 and 2.5, respectively, which do not provide them with sufficient stability towards hydrogen chloride.
One type of zeolite with a high silica-to-alumina ratio which is used to remove trace water from acid gases is known as the type AW-300 molecular sieve, which is commercially available from UOP. AW-300 is a natural mordenite-type zeolite, which has the structure M2O.Al2O3.10SiO2.6H2O, M being an alkali metal such as Na; a silica-to-alumina ratio of 10, and a pore size of 4 angstroms. This type of mordenite has been reported as useful for removing water from gas mixtures containing hydrogen chloride, such as reformer recycle hydrogen, flue gas, chloroform, trichloroethylene, vinyl chloride, and chlorine. Collins, J. J., xe2x80x9cA Report On Acid-Resistent Molecular Sieve Types AW-300 and AW-500,xe2x80x9d Molecular Sieves Product Data Sheet, Union Carbide International Co., 270 Park Avenue, New York, N.Y. 10017. Regeneration of the zeolite is accomplished by desorbing the water by purging with a hot gas at 300-600xc2x0 F. (150-315xc2x0 C.). Id See also xe2x80x9cMethod for Dehydrating Butadiene-Hydrogen Chloride Mixture,xe2x80x9d Japanese Kokai 77 89,602 (Cl. C07C11/16) Jul. 27, 1977 [c.f. CA 87:202855q]. Activated synthetic mordenite has also been reported to be useful for drying hydrogen chloride. xe2x80x9cPurification of Acidic Gases By Synthetic Mordenite,xe2x80x9d Japanese Kokai Tokyo Koho JP 61 54,235 [86 54,235] [c.f. CA 105:8642t]; xe2x80x9cZeolite For Purification of Chlorine or Hydrogen Chloride for Semiconductor Use,xe2x80x9d Japanese Kokai 77 65,194 (cl. C01B7/02), May 30, 1977 [c.f. CA:87:103913a].
The acid-resistant mordenite-type zeolites such as AW-300 have an advantage over chlorosilylated alumina purifiers in that they are stable against alumina leaching due to the fact that the zeolite structure contains isolated tetrahedral AlO2 units residing within a tetrahedral silica matrix. These units create water adsorption sites that are related to the ion exchange properties and capacity of the zeolite. In contrast, the alumina of chlorosilylated alumina is octahedral and has Alxe2x80x94Oxe2x80x94Al chemical linkages which are more vulnerable to attack and destruction by acid gases.
While the high silica mordenites have certain advantages over chlorosilylated aluminas, they are not without disadvantages. Chlorosilylated aluminas purify by an irreversible chemical reaction of surface chlorosilyl groups with water, while high silica mordenites purify primarily by physical adsorption of the water, which is a reversible process. As a result, only a small amount of water can be removed from the gas during purification over high silica mordenites before water desorption becomes significant. In addition, since the efficiency of water removal by physical adsorption is lower than that of a chemical reaction, the high silica mordenites are less effective than chlorosilylated aluminas under parallel conditions.
Although the high silica mordenites do not suffer from the problem of alumina leaching associated with chlorosilylated aluminas, these zeolites typically generate unacceptable levels of metallic impurities when exposed to acid gases. These undesirable metallic emissions are less of a problem in the case of a synthetic mordenite with lower metallic impurities. However, these zeolites still possess metallic impurities which may be emitted under certain conditions, especially when the zeolite is used to purify corrosive gases, such as HCl, in particular high-pressure HCl. Thus, in wafer fabrication, ppm-level titanium impurities in HCl, originating from the zeolite purifier, have been blamed for xe2x80x9chazexe2x80x9d and xe2x80x9cspikesxe2x80x9d observed on wafers after polishing.
Therefore, there is a need in the art for an acid gas resistant zeolite material that is capable of removing trace water from acid gases to very low levels, which does not generate unacceptable levels of metallic impurities, especially titanium, when exposed to acid gases, and which does not require moisture-generating, expensive preconditioning with the acid gas to remove these metallic contaminants. Further, there is a need in the art for a zeolite for removing trace water from acid gases suitable for both applications at low pressure, and at full cylinder pressure. In addition, there is a need in the art for a zeolite for removing metallic impurities from acid gases.
Other methods of removing water contamination from acid gases which do not rely on the use of zeolites have been reported. For example, U.S. Pat. No. 4,844,719 to Toyomoto et al. discloses a method for desiccating a water-containing gas such as hydrogen chloride which comprises contacting the gas with one side of a permeable polymer membrane made of a fluorine type copolymer, and either bringing a dry purge gas into contact with the other side of the membrane, or reducing the pressure on the other side of the membrane, thereby removing water from the gas.
U.S. Pat. No. 4,925,646 to Tom et al. discloses a process for drying a gaseous hydrogen halide such as HCl, HBr, HF or HI. The method comprises contacting the gaseous hydrogen halide with an alkylated precursor composition which comprises metal alkyl compounds dispersed throughout a support and/or metal alkyl pendant functional groups covalently bound to a support. The gaseous hydrogen halide reacts with the metal alkyl to form a scavenger composition comprising the corresponding metal halide. The method further comprises contacting the scavenger composition with the water impurity-containing gaseous hydrogen halide, causing the metal halide to react with the water impurity to yield the corresponding hydrates and/or oxides of the metal halide, and recovering an essentially water-free gaseous hydrogen halide having a water concentration of below 0.1 ppm by volume.
U.S. Pat. No. 4,564,509 to Shealy et al. discloses a method for removing oxygen, water vapor and other oxygen bearing gas species from reactant gases by bubbling the reactant gas through liquid phase ternary melt of gallium-indium and an active gathering material selected from calcium, lithium, aluminum or magnesium. The oxygen in the gas reacts with the active gathering material, forming oxides. The method may be used to remove oxygen and water from hydrogen chloride.
U.S. Pat. No. 4,663,052 to Sherman et al. discloses the use of chabazite which contains a potassium, rubidium or cesium cation in a method to dry xe2x80x9cacidic streams,xe2x80x9d such as reformer recycle hydrogen, flue gas, chloroform, trichloroethylene, vinyl chloride, chlorine, and generated hydrogen, which contain HCl as an acid component (col. 5, lines 5-17). The chabazite adsorbent of Sherman et al. is activated at a temperature of 100xc2x0 C. or greater, preferably between 200xc2x0 to 600xc2x0 C., in the presence of air or other gases (col. 5, lines 51-58).
However, these methods can result in contamination of the gas with other impurities such as oxides or metals. Moreover, these methods are generally not practical on the large scale required in semiconductor manufacturing.
Thus, there is a need in the art for a material that is capable of removing trace amounts of water from an acid gas, which is resistant to reaction with the acid gas, and in particular, which is resistant to leaching of aluminum and other metalsxe2x80x94including trace metals such as transition metalsxe2x80x94and to physical degradation. Further, there is a need in the art for a moisture-absorbing material which is capable of removing trace amounts of water from an acid gas at low as well as high pressure, which does not require a time consuming, expensive preconditioning step with the acid gas during which water is initially generated, with a concomitant temperature increase. In addition, there is a need in the art for a material which is capable of removing metallic impurities from an acid gas.
The present invention encompasses a method for removing water from a gas, in particular from an acid gas such as hydrogen chloride or hydrogen bromide, which comprises contacting the gas with a zeolite having a high silica-to-alumina ratio, wherein the zeolite has been heated prior to contacting it with the gas to a temperature above about 650xc2x0 C. Preferably, such heating is sufficient to remove an amount of water from the zeolite, believed to be produced by dehydroxylation of the zeolite, but insufficient to cause chemical and physical destruction of the zeolite.
The present invention is also directed to a method for removing water from a gas comprising heating a zeolite of high silica-to-alumina ratio and of low contaminant metal levels to a temperature of about 400xc2x0 C. for a period of time sufficient to desorb a first amount of water that is physically adsorbed onto the zeolite, wherein the zeolite has a high silica-to-alumina ratio, followed by heating the zeolite to a temperature above about 650xc2x0 C. for a period of time sufficient to remove a second amount of water from the zeolite, believed to be produced by dehydroxylation of the zeolite, thereby forming a superheated zeolite, and subsequently contacting the superheated zeolite with the gas, and allowing the superheated zeolite to absorb the water from the gas.
The present invention also relates to a superheated zeolite, which has been heated to a temperature above about 650xc2x0 C. sufficient to remove an amount of water from the zeolite, which is believed to be removed from the zeolite by dehydroxylation, wherein the superheated zeolite has a high silica-to-alumina ratio, and low metals content.
The present invention is also directed to a method of preparing a dehydroxylated zeolite, comprising heating the zeolite to a temperature of about 400xc2x0 C. for a period of time sufficient to desorb a first amount of water that is physically adsorbed on the zeolite, followed by heating the zeolite to a temperature of above about 650xc2x0 C. for a period of time sufficient to remove a second amount of water from the zeolite by dehydroxylation of the zeolite.
The present invention is also directed to a method of preparing a zeolite having a low concentration of metallic impurities, wherein the zeolite has a silica-to-alumina ratio of above about 10, comprising heating the zeolite to a temperature above about 400xc2x0 C. for a period of time sufficient to desorb water that is physically adsorbed on the zeolite, and contacting the zeolite with an acid gas for a period of time sufficient to reduce the metallic impurity concentration in the zeolite to less than about 70 ppm. The method optionally further comprises heating the zeolite to a temperature above about 650xc2x0 C. for a period of time sufficient to remove a second amount of water from the zeolite by dehydroxylation of the zeolite. The method also optionally comprises the step of cooling the zeolite to a temperature less than about 300xc2x0 C., preferably to ambient temperature, prior to contacting the zeolite with the gas. The present invention is also directed to a zeolite prepared by this method. In particular, the present invention is directed to a zeolite which has a silica-to-alumina ratio of above about 10, wherein the zeolite has been first heated to a temperature above about 400xc2x0 C. for a time sufficient to cause desorption of water that is physically adsorbed on the zeolite and then cooled to ambient temperature and subsequently contacted with an acid gas, and wherein the zeolite has a titanium concentration of less than about 20 ppm, an iron concentration of less than about 100 ppm, and a magnesium concentration of less than about 11 ppm.
The present invention is also directed to a method of removing metallic impurities from a gas, particularly an acid gas, comprising heating a zeolite to a temperature above about 400xc2x0 C. and contacting said gas with the zeolite, wherein the zeolite has a silica-to-alumina ratio of above about 10 and a low concentration of metallic impurities, and contacting said gas with the zeolite. The method optionally further comprises heating the zeolite to a temperature above about 650xc2x0 C. prior to contacting it with said gas. The method also optionally comprises the step of cooling the zeolite to a temperature less than about 300xc2x0 C., preferably to ambient temperature, prior to contacting the gas with the zeolite.